Diff compare button not showing

I believe this is because the module does not currently support anything other than diffs for Node entities. This can be seen by the fact that the diff.routing.yml file is designed around Node entities:

diff.revisions_diff:
  path: '/node/{node}/revisions/view/{left_revision}/{right_revision}/{filter}'
  defaults:
    _controller: '\Drupal\diff\Controller\NodeRevisionController::compareNodeRevisions'
    _title: Diff General Settings
  requirements:
    _entity_access: 'node.view all revisions'
  options:
    _admin_route: TRUE

I was able to add basic support in my custom entity by:

1. Adding a routing.yml entry modeled after the one above
2. Copying the contents of Drupal\diff\Controller\NodeRevisionController into my own controller and refactoring/renaming for my custom entity.
3. Copying the contents of src/Form/RevisionOverviewForm.php and refactoring/renaming for my custom entity